This paper proposes a new approach to automatic singing voice rectification. There are two components in the rectifier; one is the recognizer based on dynamic time warping and the other is the synthesizer based PSOLA (Pitch Synchronous Overlap and Add) for pitch shifting. The purpose of the recognizer is to identify the locations of off-key parts of the user's acoustic input. Then with the target music score, the synthesizer tries to correct the off-key parts by appropriate pitch shifting to match the give music score. We also attempt some singing and listening experiments for evaluating the feasibility of the rectifier and the results exhibit the satisfactory performance.